Transmission line termination



Aug. 16, 1938. l. J. KA'A'R TRANSMISSION LINE TERMINATION FilcLSept. 13,1935 Patented Aug. 16, 1938 UNITED STATES TRANSMISSION LINE TERMINATIONIra J. Kaar, Stratford,

Conn, assignor to General Electric Company, a corporation of New YorkApplication September 13, 1935, Serial No. 40,435

5 Claims.

My invention relates to high frequency transmission lines and moreparticularly to transmission lines of the concentric type.

It has for one of its objects to provide an im- 5 proved impedancenetwork for use in connection with. such lines.

A concentric tube transmission line is commonly used for conveying powerfrom an ultra high frequency transmitter to an antenna. In order toreduce the losses on such a line to a minimum, the impedance lookingfrom the end of the line into the load must equal the surge impedance ofthe transmission line. One method of matching the surge impedance of thetransmission line with the impedance of the load is to place aninductance in series with the line and a capacitance in shunt across theline, on one side or the other of the inductance, depending upon whichof the two impedances to be matched is the greater, these elements beingproportioned, in the Well known Way, to effect the desired impedancematch. Where ultra high frequencies are used the value of the capacitivereactance and the inductive reactance required are relatively small. Forexample, where the transmission frequency is 44 megacycles, thetransmission line impedance is '72 ohms, and the antenna load is 5000ohms, then the value of capacitive reactance necessary is approximately5.9 micro- 30 microfarads and the value of inductive reactance necessaryis approximately 2.16 microhenries. By suitably proportioning therelative diameters of the conductors of a concentric transmission lineat the load terminal of the transmission line 35 a sufficient shuntcapacity and series inductance may be introduced at the end of thetransmission line to permit proper surge impedance termination to beachieved.

Another object of my invention is to provide 40 an improved means forterminating a concentric transmission line.

It is a further object of'my invention to provide a shunt capacity and aseries inductance in a concentric tube transmission line by suitablyproportioning the transmission line conductors themselves.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which Fig. 1 of the drawing is acutaway view of the antenna end of a transmission line and Fig. 2 is theequivalent circuit diagram of the transmission line termination.

Referring to Fig. 1 of the drawing, I have illustrated a transmissionlineconstructed in accordance with my invention, and. which comprisestwo concentric conductors I and 2. An extension 3 of conductor 2 beyondthe end of conductor 1 forms the antenna to which the transmission lineis connected. In the particular embodiment of the invention shown, theinner conductor 2 is provided with a constricted section 4 and an en.-larged section 5 at the antenna end of the transmission line. By thusconstricting a portion of the inner conductor 2 a small amount of seriesinductance is added to the transmission line. By enlarging the innerconductor 2 as at 5 a small amount of shunt capacity is placed acrossthe line. The degree of constriction and the length of the constrictedsection 4 determines the value of the series inductance which isintroduced. The relative enlargement and the length of the enlargedsection 5 determines the value of the shunt capacity which is introducedacross the transmission line. By properly selecting the dimensions ofthe constricted section 4 and the enlarged section 5 of the innerconductor 2, the impedance looking toward the end of the transmissionline and into the load may be made equal to the surge impedance of thetransmission line.

The equivalent circuit of a transmission line termination of this typeis shown in Fig. 2. Resistance R represents the load placed on thetransmission lines I and 2 by an antenna 3. R indicates the ohmicimpedance of the transmission line itself. X1 represents the seriesinductance placed in the line by the constricted section 4 of the innerconductor 2 while X2 represents the capacity placed across the line bythe enlarged section of the inner conductor 2.

Dimensions for the proper termination of any given concentric tubetransmission line may be determined from the following well knownformulae which express the inductance and capacitance of two concentrictubes having an air dielectric.

log a where,

L=henries per centimeter, C=farads per centimeter, D=inside diameter ofthe outer tube, d=outside diameter of the inner tube.

As an example of an application of the above formulae, assume that aconcentric tube transmission line having a surge impedance of '78 ohmsis to be terminated in a vertical antenna. Assume further that theoutside diameter of the inner tube is 2.54 centimeters and that theinside diameter of the outer tube is 9.327 centimeters; that thevertical antenna has a length approximately 1.75 meters and an outsidediameter of 2.54 centimeters; and that the operating frequency is 853x10cycles. Under these conditions, the antenna will present a resistiveload of 550 ohms to the transmission line. The problem, therefore, is tomatch the '78 ohm transmission line to a 550 ohm load.

In order to match correctly the surge impedance of the transmission lineto the antenna it can readily be shown that if a network such as thatillustrated in Fig. 2 be connected between a source having a resistanceR and a load having a resistance R and if the values X1 and X2 beadjusted as follows:

and

and

It will thus be seen that the constricted section 4 of Fig. 1 must be soproportioned as to have an excess of inductance of .356 10- over thatwhich would be obtained if the inner tube were continued at uniformsize. In order that transmission line eifects will be minimized in theconstricted section, the length of the constricted section should bemade as short as practicable. Assume that a length of 100 centimeters ischosen as the length of constricted section 4. The inductance of thenormal transmission line per centimeter (from Equation (1) above) is2.6l 10" The inductance of 100 centimeters of a normal section of thetransmission line is .261 10 Therefore, the total inductance requiredfor 100 centimeters of constricted section 4 is .26l 10- +.356 10 :.61710- The inductance per centimeter of this section is .0061'7 10 ApplyingEquation (1), it will be found that the outside diameter of the innertube over the constricted portion 4 is .425 centimeters when the lengthchosen for section 4 is 100 centimeters.

The required excess capacity (as determined from Equation (3)) is 8.2810- Assume that this excess is to be obtained over a length of 25centimeters. From Equation (2) the normal capacity of the transmissionline per centimeter is xi25 10- and for 25 centimeters is 10.625 10*.Since the excess capacity to be achieved over this length is 8.28 x l0the total capacity required is 18.905 10- and the required capacity percentimeter is .756 10- Applying Equation (2) it will be found that theoutside diameter of the inner tube for the enlarged section is 4.49centimeters.

As previously stated, the amount of shunt capacity which is placedacross the line and the amount of inductance added in series to the linedepends upon the relative diameters of outer conductor I and innerconductor 2. It will, therefore, be apparent that if upon applying theabove equations for a given application the diameter of the innerconductor becomes unduly small from the standpoint of commercialapplication that the diameter of the outer conductor may be enlarged atthe point where it is desired to add series inductance rather thandecreasing the diameter of the inner conductor, or both the outerconductor may be enlarged and the inner conductor constricted.

It is frequently convenient to support the antenna at the. point whereit leaves the transmission line by a suitable insulator 6. The effect ofinsulator 6 is to introduce a certain amount of lumped capacity acrossthe transmission line. This lumped capacity may be used as a part of thecapacity provided by the enlarged portion 5 of the inner conductor 2 toprovide the proper shunt capacity.

Although I have described an embodiment of my invention wherein theimpedance of the load is greater than the surge impedance of the line,it is apparent that where the surge impedance is greater than the loadimpedance I may match the two impedances by placing the capacitance X2on the input side of inductance X1. By reversing the position of theconstricted section 4 and the enlarged section 5 in Fig. 1 such a resultmay be attained.

It will be further understood that my inven tion is not limited toimpedance matching networks but that the conductors of the concentrictransmission line may be suitably formed, in accordance with myinvention, inherently to form the elements of impedance networks ofother types as well, as for example, those used to determine thefrequency characteristic of the transmission line.

While I have shown a particular embodiment of my invention, it will ofcourse be understood that I do not wish to be limited thereto sincedifferent modifications may be made, and I therefore contemplate by theappended claims to cover all such modifications as fall within thetruespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:-

1. In a system for matching the impedance of a concentric transmissionline to the impedance of a load connected thereto, said load impedancebeing different from the impedance of said line, the combination withsaid line of a portion thereof adjacent to said load comprising aplurality of sections, one of said sections being proportioned to offerseries inductance per unit of length greater than the series inductanceper unit of length of the line and another of said sections beingproportioned to offer shunt capacity per unit of length greater than theshunt capacity per unit of length of said line, said different sectionsbeing so proportioned relative to each other that the impedance lookinginto said portion from either end thereof matches the impedance lookingin the opposite direction from the same end.

2. In a concentric tube transmission line extending between a highfrequency apparatus and an antenna, said transmission line comprising anouter conductor and an inner conductor, means including a section ofsaid inner conductor constricted with respect to normal for addingseries inductance to said transmission line, and means including asection of said inner conductor enlarged with respect to normal foradding shunt capacitance to said transmission line, said sections beingadjacent and positioned at the end of said line.

3. A concentric tube transmission line extending between a source ofoscillations and an antenna, said line comprising an outer conductor andan inner conductor, said inner conductor being connected to said antennathrough an impedance matching portion of said inner conductor, saidportion having a constricted section and an enlarged section adjacentthereto and in proximity to said antenna.

4. A concentric tube transmission line extending between a highfrequency apparatus and an antenna, said transmission line comprising anouter conductor and an inner conductor, said transmission line having asection in proximity to said antenna Where the difference in thediameter of said outer conductor and said inner conductor is increasedwith respect to normal and a second section where the difference in thediameter of said outer conductor and said inner conductor is decreasedwith respect to normal.

5. In a system for matching the impedance of a concentric transmissionline to the impedance of a load connected thereto, said load impedancebeing different from the impedance of said line, the combination withsaid line of a portion thereof adjacent said load comprising a pluralityof sections, said sections being proportioned relative to each other toconstitute the electrical equivalent of an impedance network matchingthe impedance of said load to that of said line, said network comprisinga series impedance element and a shunt impedance element, each sectionof said portion corresponding respectively to one of said elements.

IRA J. KAAR.

